Hindu worship God along with all his gifts which is given to human beings as a source of life like Air for breathing (oxygen),River for water, Sun for light, energy & whole grains(rice, pulse, etc.. in the name of Goddess Annapurna) etc.
In ancient times, Indians were largely dependent upon milk & milk products like paneer, ghee, chhachh, dahi etc. because Hindu were mostly vegetarian. Very limited vegetarian foods were available in india that time. Most of the other common vegetables that are being used in India were introduced very late. For instance:
Tomato- From Latin America (Mexico to Peru)
Potato- South America (Peru/Bolivia)
Fenugreek (Methi)-Middle East (possibly Iraq)
That is the reason major stake of life was dependent upon cow. Not only for food, cow by products like dung was used as fuel for cooking, lighting fire & fertilizer. Even it’s urine was used in life saving drugs.
Therefore Hindu worship cow like mother and pay respect to god for such life saving gifts. The total No. of Hindu God is 33 crore because it includes all the rivers which flows through the nations of India. Like Ganga, Yamuna, Godavari etc., mountains, plants like Holy Basil(tulsi) even land and many more and all are worshiped like mother because birth(life) starts from mother.